1. Field of the Invention
The present invention relates to a quantitative liquid sampling instrument for taking out an extremely small quantity of a liquid sample, a liquid reagent or the like quantitatively.
2. Description of the Related Art
In the microanalysis of biological samples, or the like, it is necessary to take out an extremely small amount of a sample or a reagent quantitatively from a container with high accuracy, and to dispense the taken sample into a reaction vessel or the like. Specially constituted systems and apparatuses are used therefor.
Usually in analysis and measurement employing such systems, only a small amount of a sample is available, or an extremely minor component like an antibody or protein in blood is determined, and furthermore, variations in quantity of the sample taken out will induce critical errors in the measurement result. Accordingly micropipets are generally used which enable the taking out of a micro-quantity of samples with high accuracy. Further, since contamination in the sample will interrupt the achievement of correct analysis data, automated apparatuses are desired for treating a number of samples successively. For the purpose of taking out and dispensing successively a number of samples, many systems are developed in which tips of the disposable type are used for the pipets of the micropipet instrument, or a tip washing step is incorporated in the system.
The micropipet instruments are operated in such a manner that the tip (or the lower end) of the pipet is inserted into a sample container, and a predetermined quantity of a liquid sample is sucked into the pipet by a negative air pressure, and subsequently the taken sample is dispensed into a reaction vessel or the like by a positive air pressure. In this operation, the pipet tip is washed, or is preferably replaced by a new one for every sample.
In the aforementioned micropipet type of quantitative liquid sampling instrument, special consideration should be given to the control of the position of the pipet tip relative to the level of the liquid sample in order to take out a liquid sample with high accuracy.
In immune reaction measurement, biochemical analysis, or the like, sample containers have generally a small opening having a diameter of as small as 15 mm or less, so that quantitative sampling of relatively viscous liquid samples like blood are greatly affected by the meniscus of the liquid. Unless the level of the liquid to be brought into contact with the pipet tip is precisely detected, the inconvenience arises that an excessively large amount of the sample may be introduced into the pipet, or a required amount of the liquid cannot be taken out precisely. When an excessive amount of the liquid sample is introduced to the pipet, the liquid sample attached to the wall of the pipet may run down during dispensation of the sample, causing disadvantageous variation in the amount dispensed. Frequently in immune reaction measurement, samples are taken out from one sample container repeatedly to utilize the whole sample effectively. In such a case, the pipet may inconveniently strike against the bottom of the sample container at the final quantitative sampling from the same container. Accordingly, the depth of dipping of the pipet needs to be detected precisely.
Provided that an "invariable quantity" of samples such as biological samples are always contained in an "invariable shape" of sample containers, it is generally understood that mere control of the descending of the pipet to a fixed position enables accurate quantitative sampling with high accuracy.
Practically, however, the quantities of a liquid sample in sample containers are rarely constant and the level of the liquid usually varies in every container, even if the container has a uniform diameter (namely having a constant horizontal sectional area). Therefore, in order to insert the pipet to a predetermined depth precisely into the liquid for quantitative sampling, it is very important in such types of quantitative liquid sampling instruments to detect the liquid level in the container for every sample and to decide the pipet inserting depth depending on the detected information.
In the fields of biochemistry and analytical chemistry, as discussed above, especially in immune reaction measurement, liquid level height (position) in a container needs to be measured for sampling quantitatively a microquantity of liquid samples, and in particular, a mechanical or automated quantitative liquid sampling instrument requires indispensably a liquid level detection device.
In the fields to which the present invention is directed, since liquid level detection technique is necessary, many specific instruments have been proposed, and developed for practical use in view of the prevention of contamination.
The liquid level detecting devices of conventional quantitative liquid sampling instruments, however, are not suitable for repeated measurements of liquid levels since the steps of liquid level detection and liquid suction are separately conducted, resulting in lower operation efficiency disadvantageously.
One example of a conventional quantitative liquid sampling instrument for biological samples in the types as mentioned above employs a non-contact type of optical liquid level detecting device which utilizes an optical reflection type of sensor, in which the distance to the liquid level is detected by bringing the sensor close to the liquid surface with downward light projection and the distance to the liquid surface is detected from the change in the measured intensity of the reflected light. This system, however, involves difficulty in achieving consistently stable measurement of various kinds of samples because the reflection from the liquid surface depends on the shape and inclination of the liquid surface, and further involves difficulty in conducting accurate measurement owing to ambiguity in detection of abrupt change of intensity of the reflected light as the liquid level detection information.
In the case where the sample in the container to be taken out is in an extremely small quantity, the diameter of the opening of the container for sucking with a pipet is small, since a large diameter of the container gives insufficient depth of the small quantity of the liquid, which causes a pipet tip to strike against the bottom of the container, preventing suction of the liquid sample. With such a small diameter of the container, it is not easy to bring the pipet and the sensor, etc. of a liquid detection device simultaneously close to the liquid face in the container, which results in a disadvantage in operation in that the liquid sucking step is necessarily separated from the liquid level detecting step.
On the other hand, liquid level detecting devices of contact type are also known which employ an electrode as the sensor, and detects the liquid level by an abrupt change of the electric current in the electrode upon contact with the liquid. Although this type of device advantageously gives a higher detection sensitivity in comparison with the non-contact type ones, the devices necessarily cause contamination by contact of the electrode with the liquid, even if a disposable type of pipet tip is employed as the pipet for the device. Therefore, such contact type devices are not suitable for microanalysis such as immune reaction measurement. Naturally, the disadvantage of contamination can be avoided by exchanging the electrode for every sample. Such a device, however, is not practical. Further, in the contact type of liquid level detecting device, the contact position of the pipet with the liquid usually deviates from the position of liquid level detection in the horizontal direction, whereby the shape or the inclination of the meniscus of the liquid sample gives much more remarkable influence in comparison with the above-mentioned non-contact optical liquid level detecting device.
For uses other than those to which the present invention is directed, another contact type of liquid level contacting device is known which utilizes the phenomenon that a ray of light introduced into a transparent body at an angle larger than the critical angle and reflected totally will be emitted outside when the transparent body is brought into contact with a different external substance. (See, for example, Japanese Patent Application Laid-Open Nos. Sho 53-55191, Sho 53-91779, and Sho 60-22730).
This type of liquid level detecting device is capable of detection at higher sensitivity in comparison with the non-contact reflection type of device using light similarly. However, in using this type of device for sampling a micro-quantity of a liquid sample, the transparent body for liquid level detection is inserted together with the pipet simultaneously into the sample container of a small diameter. This simultaneous insertion is difficult, so that the liquid level detection and the liquid suction have to be conducted disadvantageously in separate and successive steps. Further, this type of device involves the problem of horizontal deviation of the liquid level detection point from the liquid sucking pipet position, and the problem of contamination by the previous samples is encountered similarly as that in the above electrode type liquid level detection device because of the contact type of detection.